Whiskey Stones, or small soapstone cubes that you use to keep your grown-up beverages chilly without diluting them,
are pretty popular, and pretty affordable to buy. If you prefer the DIY
approach, or want to make them for friends (or make your own and sell
them), they're easy to make, too.

Whiskey stones are small soapstone cubes that are kept in the freezer until it's time to pour…
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You'll need the soapstone of course, which Instructables user alaskantomboy notes is easily available at eBay, or from construction surplus stores, salvage stores, or countertop manufacturers
pretty affordably. Once you have the soapstone, all you need to do is
measure out your cubes, mark them with a sharpie, and start cutting. The
nice thing about soapstone is that it's soft enough to cut easily,
although it has a tendency to also crack if you go to fast.

Architectural salvage stores—or the places where perfectly good hardware like doors, sinks,…
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Alaskantomboy
used a metal cutting blade on a standing chop saw to do the trick, but
also notes that if you don't have one, it's soft enough to cut with a
simple hacksaw, you'll just have to go a little slower. Once you're
finished, just use some sandpaper to smooth out the sides, round off the
edges (so you don't break your glasses when you drop them in), and
you're all finished. It's really easy, and while we can't suggest going
through the trouble when you could just buy a pack of them for $20, if you want a ton of them, this is a good way to go.